Low-emissivity coatings are well known. Typically, they include one or more infrared-reflective films each between two or more transparent dielectric films. The infrared-reflective films, which are typically conductive metals such as silver, reduce the transmission of radiant heat through the coating. The transparent dielectric films are used to reduce visible reflectance and to control other coating properties, such as color.
Low-emissivity coatings can be engineered to provide desired shading properties. As is well known, the solar heat gain coefficient (SHGC) of a window is the fraction of incident solar radiation that is admitted through a window. There are a number of applications where low solar heat gain windows are of particular benefit. In warm climates, it is especially desirable to have low solar heat gain windows. For example, solar heat gain coefficients of about 0.4 and below are generally recommended for buildings in the southern United States. Further, windows that are exposed to a lot of undesirable sun benefit from having a low solar heat gain coefficient. For example, windows on the east or west side of a building tend to get a lot of sun in the morning and afternoon. For these and various other applications, the solar heat gain coefficient of a window plays a vital role in maintaining a comfortable environment within the building. Thus, it can be very beneficial to provide windows and other glazings with coatings that establish a low solar heat gain coefficient (i.e., high shading ability coatings).
A tradeoff is sometimes made in high shading ability coatings whereby the film compositions and/or thicknesses selected to achieve a low SHGC have the effect of restricting the visible reflectance to a higher level than is ideal. As a consequence, windows bearing these coatings may have a somewhat mirror-like appearance. In addition, the transmitted and reflected colors of conventional high shading ability coatings may not be ideal. For example, these coatings may exhibit hues that are more red than is desired. The chroma of these coatings may also be greater than is desired. In most cases, it is preferable to provide a coating that is as color neutral (i.e., colorless) as possible. Thus, the reflected and transmitted colors of conventional low solar heat gain coatings may be less than ideal, both in terms of hue and chroma.
It is challenging to consistently produce any low-emissivity coating that has the desirable properties described herein. Some low-emissivity coatings have been provided with thick metal blocker layers over respective silver films. These thick blocker layers are used to establish shading performance while protecting the silver films and maintaining a balance of other properties, such as color. It has been discovered, however, that transmitted and reflected colors may be somewhat difficult to control when thick metal blocker layers are used. While production can be controlled so as to yield products with good color properties, it would be desirable to provide high shading ability coatings that are particularly easy to manufacture with consistently good color properties.
Glass tempering typically involves heating the glass to elevated temperatures on the order of 680 degrees C. Low-emissivity coatings (including those with thick metal blocker layers) may change significantly during tempering. To accommodate the changes that occur during tempering, low-emissivity coatings are sometimes provided in pairs including a temperable version and a non-temperable version. In such cases, the film stack of the temperable version is different from the film stack of the non-temperable version. These coatings are designed so the temperable version, after tempering, has properties matching those of the non-temperable version.
Further, some existing temperable coatings have limitations in terms of their durability prior to tempering. While this can be managed by adopting appropriate handling and storage practices, it would be desirable to provide temperable high shading ability coatings that offer particularly good durability prior to tempering.
It would be desirable to provide a low-emissivity coating that can achieve the following features: shading performance, low visible reflectance, pleasing color characteristics (including color stability, which is described below), good durability (including good durability for non-temperable products, good durability prior to tempering for temperable products, and good durability after tempering for temperable products), and good thermal stability (e.g., in the case of temperable products).